Liquid crystal display devices have been used for watches and electronic calculators, various household electric appliances, measuring apparatuses, automotive panels, word processors, electronic notebooks, printers, computers, televisions, etc. Typical examples of a liquid crystal display mode include a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a DS (dynamic light scattering) mode, a GH (guest-host) mode, an IPS (in-plane switching) mode, an OCB (optically compensated birefringence) mode, an ECB (electrically controlled birefringence) mode, a VA (vertical alignment) mode, a CSH (color super-homeotropic) mode, a FLC (ferroelectric liquid crystal), and the like. Also, multiplex driving is popularized as a driving method instead of usual static driving, and a simple matrix mode and a recent active matrix (AM) method of driving by TFT (thin-film transistor), TFD (thin-film diode), or the like become the mainstream.
As shown in FIG. 1, a general color liquid crystal display device includes two substrates (1) each having an alignment film (4), a transparent electrode layer (3a) serving as a common electrode and a color filter layer which are disposed between one of the alignment films and the substrate, and a pixel electrode layer (3b) disposed between the other alignment film and the substrate, the substrates are arranged so that the alignment films thereof face each other, and a liquid crystal layer (5) is held between the substrates.
The color filter layer is composed of a black matrix and a color filter including a red color layer (R), a green color layer (G), a blue color layer (B), and, if required, yellow color layer (Y).
Liquid crystal materials constituting liquid crystal layers have undergone a high level of impurity control, because impurities remaining in the materials significantly affect electric characteristics of display devices. In addition, with respect to materials constituting alignment films, it has already been known that an alignment film is in direct contact with a liquid crystal layer, and impurities remaining in the alignment film are moved to the liquid crystal layer and affect the electric characteristics of the liquid crystal layer, and thus characteristics of a liquid crystal display device due to impurities in an alignment film material have been being investigated.
On the other hand, with respect to materials such as organic pigments and the like used in the color filter layers, like the alignment film materials, it is supposed that impurities contained affect the liquid crystal layers. However, an alignment film and a transparent electrode are interposed between the color filter layer and the liquid crystal layer, and thus it has been considered that the direct influence on the liquid crystal layer is greatly smaller than that of the alignment film material. However, the alignment film generally has a thickness of as small as 0.1 μm or less, and the transparent electrode, for example, even a common electrode used on the color filter layer side and having a thickness increased for enhancing conductivity, generally has a thickness of 0.5 μm or less. Therefore, the color filter layer and the liquid crystal layer are not put in a completely isolated environment, and the color filter layer has the possibility of developing display defects such as white spots, alignment unevenness, image sticking, and the like due to a decrease in voltage holding ratio (VHR) of the liquid crystal layer and an increase in ion density (ID) which are caused by impurities contained in the color filter layer through the alignment film and the transparent electrode.
As a method for resolving the display defects due to impurities contained in pigments which constitute a color filter, there have been studied a method of controlling elusion of impurities into a liquid crystal by using a pigment in which a ratio of an extract with ethyl formate is decreased to a specified value or less (Patent Literature 1), and a method of controlling elusion of impurities into a liquid crystal by specifying a pigment in a blue color layer (Patent Literature 2). However, these methods are not much different from a method of simply decreasing impurities in a pigment, and are thus unsatisfactory for improvement for resolving the display defects even in the present situation in which a pigment purifying technique has recently been advanced.
On the other hand, with attention paid to a relation between organic impurities contained in a color filter and a liquid crystal composition, there are disclosed a method of specifying a hydrophobic parameter of liquid crystal molecules contained in a liquid crystal layer to be equal to or higher than a predetermined value, the hydrophobic parameter representing insolubility of the organic impurities in the liquid crystal layer, and a method of preparing a liquid crystal composition containing a predetermined ratio or more of a liquid crystal compound having —OCF3 groups at the ends of liquid crystal molecules because the —OCF3 groups at the ends of liquid crystal molecules have a correlation to the hydrophobic parameter (Patent Literature 3).
However, these cited documents each disclose an invention based on the principle, that the influence, of impurities in a pigment on a liquid crystal layer is suppressed and disclose no research on a direct relation between a structure of a liquid crystal material and a structure of a colorant such as a dye/pigment or the like used in a color filter, not leading to the resolution of the problem of display defects in advanced liquid crystal display devices.